multiple function args get put on their own line even with very long line length

[dhalbert]

Describe the bug
Even when the line length would permit a single-line function definition, the args are separated from their surrounding parentheses

To Reproduce
black -l 1000 on this file always puts the f4 args on their own line:

def f3(this_is_a_very_long_name, this_is_a_very_long_name, this_is_a_very_long_name):
    pass


def f4(
    this_is_a_very_long_name, this_is_a_very_long_name, this_is_a_very_long_name, this_is_a_very_long_name,
):
    pass

Expected behavior

def f3(this_is_a_very_long_name, this_is_a_very_long_name, this_is_a_very_long_name):
    pass


def f4(this_is_a_very_long_name, this_is_a_very_long_name, this_is_a_very_long_name, this_is_a_very_long_name):
    pass

Environment

• Version: 19.10b0
• OS and Python version: Ubuntu 20.04, Python 3.8.2

Does this bug also happen on master?
Yes (via a clone/virtualenv test).

I assume this is not really a bug. So I must be missing some rule about arguments I cannot find in PEP8 or the black documentation.

[ichard26]

This is not a bug, in fact this is a feature called the "magic trailing comma". It was introduced via #826... so before the release of 19.10b0. When you want Black to always explode a collection literal even if it doesn't violate the line length limit, put a trailing comma and tada, Black will explode it. (Side-note: this feature is still needs more work to fully usable, e.g. nested collection literals are unsupported ATM). So if you remove the trailing comma, Black will collapse the function signature:

(black) richard-26@ubuntu-laptop:~/programming/oss/black$ diff -u --color original.py test.py
--- original.py	2020-08-02 18:59:34.595331550 -0400
+++ test.py	2020-08-02 18:52:29.429873691 -0400
@@ -3,6 +3,6 @@
 
 
 def f4(
-    this_is_a_very_long_name, this_is_a_very_long_name, this_is_a_very_long_name, this_is_a_very_long_name,
+    this_is_a_very_long_name, this_is_a_very_long_name, this_is_a_very_long_name, this_is_a_very_long_name
 ):
     pass
(black) richard-26@ubuntu-laptop:~/programming/oss/black$ black test.py --diff --color -l 1000
--- test.py	2020-08-02 22:52:29.429874 +0000
+++ test.py	2020-08-02 23:00:07.137429 +0000
@@ -1,8 +1,6 @@
 def f3(this_is_a_very_long_name, this_is_a_very_long_name, this_is_a_very_long_name):
     pass
 
 
-def f4(
-    this_is_a_very_long_name, this_is_a_very_long_name, this_is_a_very_long_name, this_is_a_very_long_name
-):
+def f4(this_is_a_very_long_name, this_is_a_very_long_name, this_is_a_very_long_name, this_is_a_very_long_name):
     pass
would reformat test.py
All done! ✨ 🍰 ✨
1 file would be reformatted.

You can read up it more here in the docs

Hopefully that helps!

---

Environment:

• Black version: master (537ea8d)
• Python version: CPython 3.8.1
• OS version: Ubuntu 18.04.04 LTS